A voltage stiff convertor is a self-commutated convertor, the direct voltage of which, at least in the short run, is substantially constant. This can be achieved, for example, with the aid of a capacitor bank connected to the direct voltage terminals of the convertor. Such a convertor may be used for reactive power compensation of an a.c. network. The convertor is then connected via a small inductance to the a.c. network, and the inductance may consist of the leakage inductance of a transformer connected between the convertor and the network. The reactive power flow between the convertor and the network may be controlled by variation of the amplitude of the alternating voltage of the convertor. Any active power flow between the convertor and the network may be controlled by variation of the phase angle between the alternating voltages of the convertor and the network.
To reduce unwanted effects on the network from the convertor, it is desirable that the alternating voltage of the convertor exhibit a good curve shape, that is, is relatively free from harmonics. One way to reduce the harmonic content of the alternating voltage is to carry out a so-called pulse width modulation of the convertor. The large number of commutations per period in such a convertor, however, gives rise to high losses, which makes such a convertor less suitable at high power ratings.
Another way of obtaining an improved curve shape is to increase the pulse number of the convertor, that is, the number of commutations per alternating voltage period. A three-phase convertor in its simplest form has the pulse number 6. A considerable improvement of the curve shape may be obtained by doubling the pulse number of the convertor. This can be obtained in a known manner by so-called twelve-pulse connections in which the voltages from two phase-shifted six-pulse convertors are combined. There are two main types of twelve-pulse connections. In a first known connection of this kind, two separate transformers are required and therefore the equipment is complicated and expensive. In the second known convertor connection of this kind, only one transformer is used but the connection has the disadvantage that circulating current will flow between the two six-pulse convertors. Special measures in the form of increased leakage reactance of the transformer or in the form of separate inductors are required to limit these circulating currents, which causes an increased complication of the convertor. Further, the circulating currents, which cannot be entirely eliminated, give rise to additional losses in the convertors and require over-dimensioning thereof.
A further way of achieving an improved curve shape is using the so-called double six-pulse connection. In this case only one single transformer is used, with an open winding for connection to two six-pulse convertors with a common direct voltage source. By an open winding is meant a winding in which both ends of each phase winding are accessible for connection externally. Each phase winding has one of its ends connected to an alternating voltage terminal of one partial convertor and its other end connected to the corresponding alternating voltage terminal of the other partial convertor. The three phases at one end of the winding system now mentioned may be considered to constitute a first three-phase system and the three phases at the other end of the winding system may be considered to constitute a second three-phase system. The difference between these two voltage systems constitutes the voltage system that is applied to the transformer. However, this known connection has the disadvantage that a zero-sequence voltage is applied to the transformer, which zero-sequence voltage will then be supplied to that alternating voltage network to which the convertor plant is connected. This zero-sequence voltage may be limited by providing the convertor plant with a zero-sequence inductor, which absorbs the zero-sequence voltage. However, such an inductor entails a considerable complication of the plant and renders it more expensive. According to another method, the transformer may be provided with an extra unwound leg, the transformer thus operating as a zero-sequence inductor. However, this entails a complication of the transformer of some significance. Further, in such a transformer the transformer winding, connected to the network, cannot be grounded at its star point, which means that the winding has to be fully insulated and thus renders the transformer expensive.
A so-called double six-pulse connection of one of the two types mentioned above is shown in FIG. 1. The two six-pulse convertors SRA and SRB are connected on their direct voltage sides to a common direct voltage source, which consists of a capacitor bank C. The alternating voltage sides of the convertors are connected, via a transformer TR and inductors LA, LB, LC, to a three-phase network N. The convertors are controlled such that their alternating voltages are displaced in phase approximately 150.degree.. The transformer has an extra unwound leg XL.
A double six-pulse connection of the second of the two types mentioned above is shown in FIG. 2. It has a zero-sequence inductor LC with three phase windings on a common core.
A third known way of obtaining a twelve-pulse function and hence an improved curve shape is by using a so-called NPC-type convertor (NPC= Neutral Point Clamped). However, such a convertor has a more complicated main circuit and comprises more semiconductor components than a traditional double six-pulse or twelve-pulse connection. Further, in such a convertor certain of the semiconductor components are loaded more severely than the others, which for a certain given load requires an overdimensioning of the first-mentioned semiconductor components and hence makes the convertor more expensive or reduces its maximum power.
In all of the known cases mentioned above, thus, a reduction of the harmonics is obtained at the cost of increased losses and/or considerable complications of either the transformer or the convertor.